Control circuit for increasing the output inpedance of a transconductor

ABSTRACT

A transconductance circuit having a high output impedance has a dual input differential transconductor ( 10 ) with first ( 14 ) and second ( 16 ) differential inputs and a differential output ( 18 ). The first differential input ( 14 ) is connected to receive a dc voltage ( 20 ). An inverting feedback path is connected between the second differential input ( 16 ) and the differential output ( 18 ). A differential difference amplifier ( 24 ) has a first input ( 26 ) connected to receive the dc voltage ( 20 ) and a second input ( 28 ) connected to the inverting feedback path. A variable resistor ( 22 ) is connected across the differential output ( 18 ). The output ( 30 ) of the differential difference amplifier ( 24 ) is connected to control the variable resistor ( 22 ) to minimize the output of the differential difference amplifier ( 24 ), thereby creating a high output impedance from said transconductor ( 10 ).

This application claims priority under 35 USC § 119(e)(1) of provisionalapplication Ser. No. 60/239,365, filed Oct. 11, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in transconductors, or the like,and methods for constructing same, and more particularly to improvementsin methods and circuits for controlling the output impedance of atransconductor.

2. Relevant Background

A transconductor is a voltage controlled differential transconductancestage or circuit. Transconductance circuits are used in manyapplications, among which, for example is in driver circuits that supplydrive power to a dc motor used to spin a disk of a hard disk drive, orthe like.

In the construction of transconductor circuits, efforts are generallymade to make the output impedance of the transconductor as high aspossible. Traditional methods use a copy of the transconductor placed inan oscillator with feedback control so that oscillation is maintained ata constant amplitude. In this condition, the transconductor outputimpedance is nearly infinite. There are problems with this approach,however. For example, in such circuit, signals from the oscillator areinevitably leaked to the target transconductor. This is, of course,undesirable.

What is needed, therefore, is a differential transconductor which has acontrolled high output impedance, and which does not leak oscillatorsignals to the target circuit.

SUMMARY OF THE INVENTION

In light of the above, therefore, it can be seen that one advantage ofthe invention is that a differential transconductor can be providedwhich has a controlled high output impedance and which does not leakoscillator signals to the target circuit.

This and other objects, features, and advantages will be appreciated bythose skilled in the art from the following detailed description, takenin conjunction with the accompanying drawings and appended claims.

According to a broad aspect of the invention, a method is presented forcontrolling the output impedance of a transconductor having first andsecond differential inputs and a differential output. The methodincludes applying a dc voltage to the first differential input of thetransconductor and to a first differential input of a differentialdifference amplifier. An inverted output of the differential output ofsaid transconductor is applied to the second differential input of thetransconductor and to a second differential input to the differenceamplifier. The output of said transconductor is applied to a variableresistor that is controlled by an output of the difference amplifier.

According to another broad aspect of the invention, a transconductancecircuit is presented which has a high output impedance. Thetransconductance circuit has a dual input differential transconductorhaving first and second differential inputs and a differential output.The first differential input is connected to receive a dc voltage. Aninverting feedback path is connected between the second differentialinput and the differential output. A differential difference amplifierhas a first input connected to receive the dc voltage and a second inputconnected to the inverting feedback path. A variable resistor isconnected across the differential output. The output of the differentialdifference amplifier is connected to control the variable resistor tominimize the output of the differential difference amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawings, in which:

FIG. 1 is an electrical schematic diagram of a transconductor andassociated circuitry to control the output impedance to near infinity,in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred embodiment of the invention, atransconductor 10, is presented, together with associated controlcircuitry 12 to control the output impedance to near infinity. Thetransconductor 10 has dual differential inputs 14 and 16, and adifferential output 18. A dc voltage 20 is applied to the firstdifferential input 14, and the output on the differential output 18 isinverted and applied to the second differential input 16. The output onthe differential output 18 is also applied to a variable resistancecircuit 22, which is a part of the control circuitry 12.

The control circuitry 12 includes, in addition to the variableresistance circuit 22, a differential difference amplifier 24, which hastwo sets of differential inputs 26 and 28, and a single output 30. Thedc voltage 20 is applied to the first differential input 26, with thesame polarity as it is applied to the first differential input 18 of thetransconductor 10. The output of the transconductor 10 is applied to thesecond differential input 28 with an inverted polarity signal applied tothe gates of input transistors 12 and 14.

The construction of the variable resistance circuit 22 is such that ithas a variable resistance that is controlled by the output signal 30from the differential difference amplifier 24. It should be noted thatin the particular application described, the variable resistance may beor include negative resistance values.

The negative feedback loop formed by the differential differenceamplifier 24 forces the difference between inputs 14 and 18 of thetransconductor 10 to be minimal. This difference is limited by the inputoffset of the differential difference amplifier 24.

With the circuit constructed as above described, the nettransconductance current is zero, since the two inputs (which have equaltransconductances) are equal in magnitude but opposite in polarity.Since net current entering any node must be zero (Kirchhoff's CurrentLaw), and since the net current entering the output nodes equals the sumof transconductance current and output conductance current, the lattermust also be zero. However, the transconductor output voltage is anon-zero DC voltage (in fact equal to the voltage of the firstdifferential input to the transconductor). Hence the impedance lookinginto the output of the transconductor must be infinite.

The impedance control feedback loop controls the input control signal tothe variable resistance circuit 22 such that its output conductancecancels the output conductance of the transconductor. This input controlsignal, which could be a voltage signal, may then be used in othertransconductors, for example, in a continuous-time filter that each hasa similar variable resistance circuit.

Although the invention has been described and illustrated with a certaindegree of particularity, it is understood that the present disclosurehas been made only by way of example, and that numerous changes in thecombination and arrangement of parts can be resorted to by those skilledin the art without departing from the spirit and scope of the invention,as hereinafter claimed.

What is claimed is:
 1. A method for controlling the output impedance ofa transconductor having first and second differential inputs and adifferential output, comprising: applying a dc voltage to said firstdifferential input of said transconductor and to a first differentialinput of a differential difference amplifier; applying an invertedoutput of said differential output of said transconductor to said seconddifferential input of said transconductor and to a second differentialinput to said difference amplifier; applying said output of saidtransconductor to a variable resistor; and controlling a resistance ofsaid variable resistor by an output of said difference amplifier.
 2. Themethod of claim 1 wherein said variable resistor presents a negativeresistance to said output of said transconductor.
 3. A circuit forcontrolling the output impedance of a transconductor having a firstdifferential input for receiving a dc voltage, a second differentialinput, and a differential output, comprising: a inverting connectionfrom said differential output of said transconductor to said seconddifferential input of said transconductor; a differential differenceamplifier having a first input connected to receive said dc voltage, asecond input connected to receive and an inverted output from saidtransconductor, and an output; and a variable resistor across which saidoutput of said transducer is connected; said output of said differentialdifference amplifier being connected to control said variable resistorto minimize the output of said differential difference amplifier.
 4. Thecircuit of claim 3 wherein said variable resistor had a negativeresistance.
 5. A transconductance circuit having a high outputimpedance, comprising: a dual input differential transconductor havingfirst and second differential inputs and a differential output, saidfirst differential input being connected to receive a dc voltage, aninverting feedback path between said second differential input and saiddifferential output; a differential difference amplifier having a firstinput connected to receive said dc voltage and a second input connectedto said inverting feedback path, and an output; a variable resistorconnected across said differential output; and said output of saiddifferential difference amplifier being connected to control saidvariable resistor to minimize said output of said differentialdifference amplifier.
 6. The method of claim 5 wherein said variableresistor has a negative resistance.